1. Field of the Invention
This invention relates to a process for preparing a photopolymerizable printing element.
2. Description of the Prior Art
Flexographic printing plates are well known for use in printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,749. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
The flexographic photopolymer compositions can be formed into sheets or layers by several known methods such as solvent casting, hot pressing, calendering and extrusion. A preferred method of manufacturing flexographic printing elements is by extrusion calendering of the photopolymer composition. In extrusion calendering, the printing element is prepared by passing a mass of hot photopolymer into an extrusion die forming a layer, passing the layer into nip of a calender and, while still hot, calendering the photopolymer composition between two flat surfaces, generally two flexible films, to form a multilayer web. The films can include multiple layers or compound films. A film bearing a thin layer of flexible, polymeric film is an example of a compound film. After extrusion and calendering at elevated temperatures, the web is held in tension in the film transport direction by a pair of nip rollers while the multilayer web is cooled, for example, with blown air. The printing element as a multilayer web can be cut into suitable size sheets.
A problem associated with the extrusion calendering of photopolymeric layers is that while the web is in tension and being cooled, non-uniformities in the thickness of a photopolymer layer can occur across the web, i.e., cross-web or transverse direction, for the length of the web in the film transport direction. For example, the web can have areas in the film transport direction where the thickness of the photopolymer layer is greater or less than desired alternating with areas where the thickness of the photopolymer layer is on target. Transverse non-uniformity in thickness of the photopolymer layer which runs the length of the web is generally referred to as laning. Non-uniform thickness of the photopolymer layer typically causes the printing element to not lie flat and thus a mask or phototool used during exposure does not contact the element properly and vacuum is not sufficient to improve contact. Flexographic printing plates thus prepared from printing elements having non-uniform thickness results in poor relief images in the plate and misregistration of printed images.
Thus, it is an object of this invention to provide an improved process for the preparation of photopolymerizable printing elements in which the thickness of the photopolymer layer does not change and is substantially uniform across the web.